Press-in fasteners such as the TackSert® fastener product line of Penn Engineering & Manufacturing Corp. have diagonal knurls. These pins can be installed into either brittle or soft material by pushing the head and allowing the knurls to enter the host material. During this installation one or a combination of two things happens: the knurls cut and dig into a more brittle host or the knurls push aside a more malleable host.
In brittle materials, the knurls will cut the host and pack the cut material into the knurl teeth creating a press-fit situation to create a tight bond between the pin and the host material. Because this packed material represents already cut and weakened pieces, the retention force is adversely affected. In a more ductile and malleable host, the displacement force pushing inward axially can fold-over and plow ahead the host instead of allowing it to flow around the knurl.
Other knurl configurations have not improved this need in the art for a more effective knurl with enhanced retention force. Straight or vertical knurls pull out axially very easily due to a lack of any mechanical means of retention. Diamond knurls cause more localized destruction of the host reducing further the retention forces. Ribbing, i.e. horizontal ‘knurls’, allows better performance in some more elastic hosts, but in brittle hosts or less elastic hosts, the retention is worse. Diagonal knurls will cause the pin to rotate during installation as the pin tries to naturally minimize the forces to install. This can cause a slight reduction in the retention force.
There is therefore a need in the fastener arts for an improved tack pin fastener that will overcome the above described deficiencies in fastener retention and be effective in a wide variety of host materials of varying hardness.